Approximately 100,000 rear impacts per year result in whiplash injuries to the vehicle occupants. Most of these injuries could be prevented if the headrest were properly positioned behind the head of the occupant and if it had the correct contour to properly support the head and neck of the occupant. Whiplash injuries are the most expensive automobile accident injury even though these injuries are usually are not life threatening and are usually classified as minor.
A good discussion of the causes of whiplash injuries in motor vehicle accidents can be found in Dellanno et al, U.S. Pat. Nos. 5,181,763 and 5,290,091, and Dellanno, U.S. Pat. Nos. 5,580,124, 5,769,489 and 5,961,182, which are included herein by reference, as well as many other technical papers. These patents discuss a novel automatic adjustable headrest to minimize such injuries. However, these patents assume that the headrest is properly positioned relative to the head of the occupant. A survey has shown that as many as 95% of automobiles do not have the headrest properly positioned. These patents also assume that all occupants have approximately the same contour of the neck and head. Observations of humans, on the other hand, show that significant differences occur where the back of some people's heads is almost in the same plane as the that of their neck and shoulders, while other people have substantially the opposite case, that is, their neck extends significantly forward of their head back and shoulders.
One proposed attempt at solving the problem where the headrest is not properly positioned uses a conventional crash sensor which senses the crash after impact and a headrest composed of two portions, a fixed portion and a movable portion. During a rear impact, a sensor senses the crash and pyrotechnically deploys a portion of the headrest toward the occupant. This system has the following potential problems:
1) An occupant can get a whiplash injury in fairly low velocity rear impacts; thus, either the system will not protect occupants in such accidents or there will be a large number of low velocity deployments with the resulting significant repair expense.
2) If the portion of the headrest which is propelled toward the occupant has significant mass, that is if it is other than an airbag type device, there is a risk that it will injure the occupant. This is especially true if the system has no method of sensing and adjusting for the position of the occupant.
3) If the system does not also have a system which pre-positions the headrest to the proximity of the occupant's head, it will also not be affective when the occupant's head is forward due to pre-crash braking, for example, or for different sized occupants.
A variation of this approach uses an airbag positioned in the headrest which is activated by a rear impact crash sensor. This system suffers the same problems as the pyrotechnically deployed headrest portion. Unless the headrest is pre-positioned, there is a risk for the out-of-position occupant.
U.S. Pat. No. 5,833,312 to Lenz describes several methods for protecting an occupant from whiplash injuries using the motion of the occupant loading the seat back to stretch a canvas or deploy an airbag using fluid contained within a bag inside the seat back. In the latter case, the airbag deploys out of the top of the seat back and between the occupant's head and the headrest. The system is based on the proposed fact that: "[F]irstly the lower part of the body reacts and is pressed, by a heavy force, against the lower part of the seat back, thereafter the upper part of the body trunk is pressed back, and finally the back of the head and the head is thrown back against the upper part of the seat back . . . . " (Col. 2 lines 47-53). Actually this does not appear to be what occurs. Instead, the vehicle, and thus the seat that is attached to it, begins to decelerate while the occupant continues at its pre-crash velocity. Those parts of the occupant that are in contact with the seat experience a force from the seat and begin to slow down while other parts, the head for example continue moving at the pre crash velocity. In other words, all parts of the body are "thrown back" at the same time. That is, they all have the same relative velocity relative to the seat until acted on by the seat itself. Although there will be some mechanical advantage due to the fact that the area in contact with the occupant's back will generally be greater than the area needed to support his or her head, there generally will not be sufficient motion of the back to pump sufficient gas into the airbag to cause it to be projected in between the head that is not rapidly moving toward the headrest. In some cases, the occupant's head is very close to the headrest and in others it is far away. For all cases except when the occupant's head is very far away, there is insufficient time for motion of the occupant's back to pump air and inflate the airbag and position it between the head and the headrest. Thus, not only will the occupant impact the headrest and receive whiplash injuries, but it will also receive an additional impact from the deploying airbag.
Lenz also suggests that for those cases where additional deployment speed is required, that the output from a crash sensor could be used in conjunction with a pyrotechnic element. Since he does not mention anticipatory crash sensor, which were not believed to be available at the time of the filing of the Lenz patent application, it must be assumed that a conventional crash sensor is contemplated. As discussed herein, this is either too slow or unreliable since if it is set so sensitive that it will work for low speed impacts where many whiplash injuries occur, there will be many deployments and the resulting high repair costs. For higher speed crashes, the deployment time will be too slow based on the close position of the occupant to the airbag. Thus, if a crash sensor is used, it must be an anticipatory crash sensor as disclosed herein.